1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to novel semiconductor light-emitting devices which can generate light with arbitrary color.
2. Related Art
Solid-state lighting is expected to be the next wave of illumination and display technologies. In addition to emerging novel applications, such as serving as the light source for display devices and replacing light bulbs for conventional lighting, light-emitting-diodes (LEDs) can be used directly for panel display. LED active display has the advantage of shorter refreshing time, larger viewing angle, and lower power consumption.
An LED produces light from an active region, which is “sandwiched” between a positively doped layer (p-type doped layer) and a negatively doped layer (n-type doped layer). When the LED is forward-biased, the carriers, which include holes from the p-type doped layer and electrons from the n-type doped layer, recombine in the active region. In direct band-gap materials, this recombination process releases energy in the form of photons, or light, whose wavelength corresponds to the energy band-gap of the material in the active region.
Depending on substrate material and the design of the semiconductor layer stack, an LED can be formed using two configurations, namely the lateral-electrode (electrodes are positioned on the same side of the substrate) configuration and the vertical-electrode (electrodes are positioned on opposite sides of the substrate) configuration. FIG. 1A illustrates the cross section of a typical vertical-electrode LED which includes a conductive substrate layer 102, an n-type doped layer 104, a multiple-quantum-well (MQW) active layer 106, a p-type doped layer 108, a p-side electrode 110 coupled to conductive substrate layer 102, and an n-side electrode 112 coupled to n-type doped layer 104.
The vertical-electrode configuration makes packaging of the device easier. In addition, because the electrodes are located on opposite sides of the device, a vertical-electrode LED is more resistant to electrostatic discharge. Therefore, vertical-electrode LEDs have a higher stability compared with lateral-electrode LEDs. This is especially true for high-power, short-wavelength LEDs.
The recent developments in LED fabrication technology enable the use of GaN-based III-V compound semiconductors, which include AlGaN, InGaN, InGaAlN, and GaN, as materials for short-wavelength LED. These GaN-based LEDs have extended the LED emission spectrum to the green, blue, and ultraviolet region.
In order to expand the light-emitting spectrum of an LED, novel methods have been proposed to include a color (wavelength) conversion layer on top of a short wavelength LED, such as a blue light LED. FIG. 1B illustrates the cross section of such a device. A color conversion layer 114 is deposited on top of an n-type layer 104 and an n-side electrode 112. Note that sometimes the color conversion layer can be part of the LED package. The inclusion of a color conversion layer with the LED can modify the LED emission spectrum. For example, an LED whose emission spectrum is originally in the blue range, which is determined by the structure of MQW, may emit red or green light after color conversion.
A color conversion layer can use fluorescent powder. Such a method utilizes the photoluminous property of a fluorescent powder, which when excited by a light emits a light of different wavelength (color) based on its chemical composition. For example, red fluorescent powder can be excited by a shorter-wavelength blue light emitted by an LED and consequently emits a red light. Similarly, when green fluorescent powder is excited by the same blue light, it emits a green light.
Various techniques have been proposed to use florescent powder to change the color of an LED, especially to produce an LED that emits white light. For example, U.S. Patent Application Publication No. 2005/0168127 has proposed disposing fluorescent powder around an exciting light source. By optimizing the composition of the fluorescent powder according to the wavelength of the exciting light source, the LED can be configured to emit white light. In addition, U.S. Patent Application Publication No. 2005/0230693 has proposed to add red florescent powder to the package material of an LED that has both a green light-emitting layer and a blue light-emitting layer to form a single chip LED with three luminescent spectrums of red, blue, and green wavelengths. However, both techniques are not suitable for forming a color panel LED display because the color of each packaged LED is predetermined and cannot be adjusted dynamically. Moreover, using a packaged LED as a single-color pixel often results in a low fill factor which can lead to undesirable “screen-door” effect on the display.